Melting point

Figure 1. Pure water has a melting point of 0°C, which allows one to test any other sample of water to check its purity.

Melting point is the temperature at which a solid will change phase into a liquid. In theory it will also be the same temperature as the freezing point, where a liquid will turn into a solid, but in practice there are slight differences. For example, in theory pure water has a melting point and freezing point at 0°C under standard temperature and pressure, but slight measurable differences tend to arise.

The transition between the solid and the liquid is so sharp for small samples of a pure substance that melting points can be measured to 0.1°C. Therefore melting points can be used to accurately measure the purity of a substance. See Figure 1 on the right.

While the solid is in the process of melting, its temperature will not change (see Figure 2 below). This is known as latent heat, which is the opposite of sensible heat.

Figure 2. This is a heating curve diagram, specifically for water. Notice how temperature (on the y-axis) stays constant during a phase change. This is because the heat absorbed (x-axis) is being used to weaken intermolecular bonds instead of accumulating as kinetic energy.

For Further Reading

• Temperature
• Phase change
• Standard conditions for temperature and pressure
• Latent heat
• Sensible heat
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